Parkinson's disease (PD) is one of the most common adult neurodegenerative disorders, affecting over one million people in North America. Although effective symptomatic treatments are available for PD, there are no medications available to forestall the inexorable clinical decline. As a first step in identifying such therapies, the NINDS Exploratory Trials in Parkinson's disease (NET-PD) network successfully completed futility studies, identifying creatine as a potential agent to slow clinical decline in PD. The NET-PD network is now conducting a large, long-term. Phase 3 trial (known as LS1) comparing creatine to placebo using a novel design and statistical approach. The aims of this competitive renewal are to complete follow-up and retention of enrolled subjects in the LS-1 study and to assess the efficacy of creatine using a novel primary composite clinical decline outcome. Secondary analyses include individual components of the primary outcome measure, use of health services, levodopa-dose equivalents, safety, and tolerability. The Washington University School of Medicine site has demonstrated outstanding retention in the NET-PD studies (97-100%) and 99% query resolution. The Washington University School of Medicine site has extensive experience with clinical trials and management of PD, making this an ideal site for this exploratory PD trials network.